BT and TalkTalk have been granted permission to appeal against their failed legal challenge to the Digital Economy Act, which is central to the government's attempts to curb illegal downloading.

The decision means that there will now be a hearing at the court of appeal – likely to be in the new year – which means that the government's plans to send thousands of warning letters to alleged illegal downloaders will probably be pushed back by about six months.

The government had planned to start sending the letters in the first half of next year, but it will now have to hold off until the challenge by Britain's two largest ISPs is resolved.

Lord Justice Lewison granted BT and TalkTalk leave to appeal the DEA on four grounds. Both companies have raised concerns about how the legislation works alongside EU directives on technical standards, authorisation, e-commerce, as well as privacy and electronic communications.

The hearing will be heard by three judges, one of whom must have EU experience – which the ISPs privately believe may help their cause.

"We are pleased to have been granted permission to appeal the high court judgment," said BT. "We now expect that the hearing will take place as soon as possible."

Today's ruling was effectively the last throw of the dice for the two ISPs after judge Sir Richard Buxton turned down the last attempt in June.